The present invention relates generally to an imaging system, and more particularly to an X-ray tomosynthesis mammography imaging system.
X-ray motion tomography is an X-ray imaging method in which an X-ray tube and a detector move in opposite directions relative to a patient or another object that is being imaged. In tomography, X-rays beams are turned on while the X-ray tube and the detector are in motion, thereby selecting a single in-focus plane and blurring out all others. However, tomography is used to image only a single in-focus slice of the object being imaged rather than a three dimensional volume of the object being imaged. To image other slices of the object, additional scans of the X-ray tube and detector are needed. If a human patient is being imaged, repeated scans cause an undesirably high X-ray exposure dose for the patient.
X-ray mammography is the modality of choice for breast cancer screening. U.S. Pat. No. 5,872,828 incorporated by reference in its entirety describes a tomosynthesis system for breast imaging. This system produces a three dimensional image of the breast being imaged from a single scan of the X-ray source in an arc shaped path. The tomosynthesis system contains an X-ray source which moves in an arc shaped path over the breast that is being imaged, a stationary digital X-ray detector and an image processor. The detector is mounted on a stationary portion of a support structure. The X-ray source is mounted on a movable portion of the support structure. The movable portion of the support structure is an arm whose lower end is rotatably attached to the stationary support structure at a pivot point, and whose upper end supports the X-ray source.
However, while this tomosynthesis system is capable of forming adequate three dimensional mammography images, it suffers from several disadvantages. In order to obtain a three dimensional image of the breast, the image that is formed from the rotation of the X-ray source in an arc shaped path is mathematically transformed to construct an approximation of an image that would have resulted as if the X-ray source moved in a line shaped path parallel to the detector plane. Thus, the image for each tomographic plane is transformed to approximate an image that would have been formed based on the so-called Twinning principle. A three dimensional image of the breast is then reconstructed from the individual reconstructed images of tomosynthesis planes based on the Twinning principle. This algorithm adds complexity to the image processing.
Furthermore, the X-ray source is subject to a large amount of vibration because the system operates in a “step and shoot” mode, which distorts and blurs the image. In this mode, the X-ray source is off during its movement along the arc shaped path and is on during stops along the path. Thus, the X-ray source moves along the path, then stops, emits an X-ray beam (i.e., emits a shot) and then continues to move. The start and stop motion of the X-ray source causes the X-ray source to vibrate during the sudden stops, which causes the X-ray beam to be displaced relative to the object and to increase the blurring of the image. Thus, a plurality of projection images from a plurality of projection angles are formed. The range of the projection angles is less than 180 degrees. Thus, the X-ray source moves in an arc shaped path in a plane of a sector of a sphere, as opposed to moving 180 to 360 degrees around the patient.